I'm looking for exercises to help maintain a grip on an opponent's gi while pulling them around. There are two elements to this: strength and endurance. Meaning, I want my grip to be hard to break but also not tire out too much when I'm holding on to it.

This, use your gi over a pull up bar. Also go jogging with bricks (literally) in your hands. Youll be amazed at the results.
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user201Feb 9 '12 at 5:56

Is it not dangerous for the back, knees or shoulders to jogging with weight in the hand?
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aberrationJul 5 '13 at 16:20

@user201 is right, but maybe start with a smaller one. The main emphasis is on holding something heavy and rough while keeping the palm open as much as possible. I started with huge coke bottles filled with sand then finally ended up with big rough bricks.
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nigelhanzoFeb 20 '14 at 10:27

We did an exercise at my first Judo club that seemed to help with grip. We would hold our arms out directly in front of us and then alternate between making a grip and having our hands as open as possible. Basically, like gripping thin air, but repeatedly. A very simple exercise, but it seemed to help.

How many times we repeated was a measure of how many were attending the session that evening, because we'd stand in a circle, facing in, and everyone would do the exercise together, but someone would count to ten and then the next person would count to ten and so forth until we'd been around the circle at least once; twice if we had a low turnout.

Aside from the strength training for grips, there's also certain technique involved in grips. For example, when you hold the right arm sleeve of your opponent, make sure you grab the part right under his elbow, and grab it tight, so he is unable to move his arm properly. At the same time it's hard for him to release his harm since you can control his arm movement.

When you grab the neck part, make sure you grab the harder edge. Your fingers grab the outer side so that the soft part gets push under the hard part. The thumb goes inside and connects with your other fingers.

Technique is really important with everything you do in Judo. Even with grips.

Basically either go outside and find something overhead or get a pullup bar and hang from it. You can make a progression out of this, going from a pullup bar to one-hand on a pullup bar to hanging from a rope. The basic idea is that if you want to improve your grip strength, grip something!

It also tends to be pretty easy to find things to hang from, making it something you can do throughout the day if your environment is conducive to it.

If you are looking for a program, Convict Conditioning 2 has one that incorporates progressively more difficult variations of this and also recommends fingertip pushups. My standard disclaimer on CC applies: it tends to have great progressions, but ignore the marketing material.

Learn Massage (and other skills)

Trigger point massage is one of the things that has helped my overall grip strength a great deal. I started with Clair & Amber Davies's The Trigger Point Therapy Workbook, but really many if not most systems of massage will help tremendously here.

Here the big thing is to learn to work with your fingers, not your thumbs, but it does help grip strength if you practice regularly. Learning has the added benefit that you can treat a wide variety of muscular issues on yourself.

There are also other sports that strongly emphasize grip strength and it may helps to get a foundation in. I don't personally practice it, but an example here might be rock climbing.

Weapons Training

Another thing I've had very good luck with is weapons training. Working with swords and short sticks (and probably any other weapon that you grip and has some mass), and particularly against a target, is a great way to build up hand strength, and is particularly useful for the "endurance" part.

The added advantage here is that it can usually be incorporated into your martial arts practice if your art uses weapons.

There are a few things you can do to improve grip strength. Like everything else, sport specific drills are best, training the closest to the actual activity your trying to improve.

on that note:

Like others have mentioned, throw a gi over a bar and do pull ups with it.

My favourite, because it works all sorts of different parts of your upper body, and can be incorporated into warmups. with a partner, one person takes off their belt, and goes down the mat far enough that the belt is stretched out. the other partner lays on their back, and "climbs" up the belt. to make it more interesting, hold your legs straight, about 6" off the mat. We used to do this drill down the full length of the dojo. once on our back, once on our stomach, and once on each side. Both partners will get a good hand workout, and the climbing/pulling partner will get a good overall upper body workout.

get a fairly firm stress ball, and play with it, all the time. squeeze it with your full hand, one finger at a time, a couple fingers. If you can't find a firm enough stress ball that it is challenging, use a tennis ball like stslavik mentioned.

Do more grip fighting, nothing will improve your grip strength and technique more then just practicing gripping.

Put a gi on the ground in the middle, one partner on each side, lying on their stomach. the goal is to get the gi away from your partner, or pull them to the other side. We did this in class the other day, it was fun, challenging, and sure made the forearms burn, which is a good sign ;)

Your goal to increase your grip strength should focus on exercising your forearm muscles. There are several strength exercises and tools you can use to work out the different muscles in your your forearms.

Tennis Ball or Hand Grip

As @stslavik stated in his comment, you can grab a tennis ball and squeeze it to your heart's content. When squeezing the tennis ball, also hold the squeeze for a few seconds before releasing. As you improve, you can increase the number of repetitions you do. Additionally you can buy hand grip tools. These aren't terribly expensive things, but keep in mind you might reach an upper limit to the amount of strength training you can do with them once you reach a certain level.

Wrist Curls and Reverse Wrist Curls

A single barbell or a dumbbell in each hand will can suffice for this. You can do this while sitting and resting your arms on your legs. Anything that will support the weight of your forearms will work really.

Hold the barbell or dumbbell with your palms facing up and just roll your wrist towards you without lifting your arm and then release back down to a relaxed position. The reverse wrist curl is the same except your palms are facing down, and you roll your wrists backwards towards you. Here are pictures of what these two exercises look like:

You could also perform the wrist curls with your arm outstretched, but depending on what your level of ability is, you might end up working to keep your arms out more than working on your wrists. If weights aren't an option for you, consider using a resistance band instead (stand on one end and pull the other end up with your wrist).

Make Your Own Wrist Roller

A simple thing you can do is buy a plate or dumbbell, tie a rope through or around it and tie the other end to a bar or rod. Hold your hands out in front of you, and begin twisting the rod so that the rope wraps around the rod as the weight goes up. When the weight reaches the top, begin untwisting and let the weight go back to the ground. You can also choose to use different weights with it if you so desire. Here's an example of what it looks like from BodyBuilding.com:

That's the basic premise of one. If you don't have plate or barbell, you can improvise with using other materials like a gallon of water or a bag of rocks. You don't need a whole lot of weight to feel it as you curl the rope up or down. A broomstick would work just fine for holding onto as you train.

Buy a Wrist Roller

A SportGrips SideWinder is a simple device that is an extremely durable, small, and portable, and has adjustable tension which is controlled by a knob at the top. I wrote a blog post about this on the Fitness & Nutrition blog. It's simple and easy to use - adjust the tension, grab, and twist. It definitely helped me increase my grip strength over the last few months. The Sierra Mega Wrist Roller is also a similar product you can use. The only downside to these type of products are the cost if money is a concern, but you get what you pay for.

i've always liked getting a bucket or small garbage can and filling it full of uncooked rice. you can do what we call rice grabs by jamming your fist into the rice while grabbing and releasing handfuls of rice. cheap and effective.

We are taught to practise in bjj to use a gi (uniform) you can hang it on something strong and practise pulling up if you do not have a gi to use for this excercise you can use a belt a martial art belt if possible.
It helps to work your arms and you will be able to hold on with that grip when the opponent is resisting or moving.

Get Strong First

If you're weak, there's no point in trying to get a strong grip. You're too weak for it to matter. Instead, you should first work on getting your whole body strong, and then--only then!--focus on developing your grip. Mark Rippetoe has some words for people lacking general strength who want to increase strength in one particular area. From Jim Wendler's blog:

Strength is the most general adaptation. It is acquired most effectively through exercises that produce the most force against external resistance, and as such is always best trained with five or six basic exercises. The same exercises that are correct for weak football players and lifters are correct for weak volleyball and baseball players, because the best way to get strong will always be the same. Strength is NOT specific, and cannot effectively be acquired through exercises that mimic sports-specific movements, because these movements lack the potential to produce as much force as general barbell exercises, and therefore lack the capacity to make weak athletes as strong as barbell training.

In other words, grip-specific training is barking up the wrong tree. Unless the student is already quite strong, the best way to develop grip strength is to work on general strength. Whole-body training will improve your grip strength faster than forearm-specific work, while giving you health benefits and strength for other movements as well.

General Training with a Focus on Grip

Keeping that in mind, what is the best way to tailor your general strength training to grip strength?

The deadlift should be your primary exercise. After an initial period of a few weeks doing higher-rep sets to get your form down, your priority should be warming up to one near-maximal set of three to five reps. Avoid the mixed grip and even the hook grip for as long as possible in your training, and once you start using them, keep doing your warm-ups without them.

Pull-ups with a gi are king. They train grip strength (particularly on the downward phase) as well as grip strength endurance when done for 10+ reps. Best of all? You'll develop enormous upper-body strength that's applicable in grappling far beyond just gripping. You can supplement this training with climbing a rope and finishing your pull-up sets with holds at the top and bottom. If pull-ups are hard for you, negatives are very useful.

Do at least a little of the counter movements to these exercises, to prevent imbalances. Dips and barbell front squats would be fine choices.

Once you can perform fifteen full-range-of-motion, no-kip gi-grip pull-ups, plus a double bodyweight deadlift, the student can add the awesome grip-specific training mentioned by others. Fat grips, oversize bars, wrist rollers, curls and other specialized training are very productive...once someone is strong enough everywhere else for it to matter.

Other approaches are of course possible: a kettlebell-centric program with many swings and clean-and-presses would appropriately develop whole-body strength while also working the grip.

Put a 3 inch vertical handle on five gallon bucket. Each day fill it with a bowl of water and lift with fingers from floor to chest for 15 minutes with each hand. In three months your hands will be three times stronger. After using water, repeat the process with sand, and then with lead. If you can lift a full bucket of lead, you can actually control any man or beast with just hand strength. It's called dragon's claw.

I have used a Bulgarian Bag for grip strength, and I recommend it. The different handles allow you to perform a variety of exercises, and there are many, many instructional videos on how to use the bags (for grips as well as other exercises). We keep a few at our academy for conditioning/competition training circuits.